In the field of medicine, imaging and image guidance are a significant component of clinical care. From diagnosis and monitoring of disease, to planning of the surgical approach, to guidance during procedures and follow-up after the procedure is complete, imaging and image guidance provides effective and multifaceted treatment approaches, for a variety of procedures, including surgery and radiation therapy. Targeted stem cell delivery, adaptive chemotherapy regimes, surgical tumour resection and radiation therapy are only a few examples of procedures utilizing imaging guidance in the medical field.
Real-time imagery of tissue being operated on may be generated by a surgical imaging system. Typically this is a sequence of images forming a video stream that is updated as a surgical operation is performed. Often there is a particular feature in the imagery that is the subject of the operation such as a glioma, white matter tracts, tendons, ligaments or muscle fibers. It may be very important for the surgeon to be able to distinguish different portions of the feature and distinguish the feature from surrounding tissue. However, while the feature may have distinguishing colour characteristics in colour imagery, it may be difficult for the surgeon to see such distinctions in a typical image or video stream.
In a stereoscopic camera system, the three-dimensional (3D) effect experienced by the viewer is determined by the inter-ocular separation and convergence angle of the camera system. As seen in FIG. 1, the inter-ocular separation is the distance (“d”) between the two cameras. The convergence angle, or parallax angle, (θ) is the angle away from perpendicular of the camera.
As the inter-ocular separation increases, the 3D depth effect increases as well. The convergence angle determines which plane in the image defines the flat, or z=0 location. Areas below this plane look like they are receding into the display, whereas areas above the plane appear as if they are ‘popping out’ of the display. A particular set of inter-ocular separation and convergence angles leads to a pleasing and realistic picture for the viewer.
The above is only strictly true at one particular magnification, working distance, display size, and position of the observer relative to the display. Fundamentally, all four of these ‘perceived’ parameters affect the perception of 3D because they change the disparity as it appears to the viewer.
For example, as the viewer moves closer to the screen, the 3D depth effect is compressed. The same is true for magnification (as you zoom in, the images and hence their separation as a percentage of the field of view increases). Changing the working distance is simply another way of changing the magnification (e.g. as the camera moves closer, the image size increases). Finally, the display size affects the perceived disparity since the smaller the display the smaller the perceived separation.
Eye fatigue with a 3D surgical microscope system is a significant challenge for surgeons who often operate for multiple hours. For example, spinal surgeons and neurosurgeons may operate for 8 to 12 hours or more in a complicated case. Moreover, visual functions vary greatly from person to person. In particular, there are limits to the binocular parallax of left and right images that a person can fuse into one 3D image in the brain. Some people can converge and generate 3D images very easily while approximately 3-15% of the population have stereo blindness or stereo impairment. A significant percentage people cannot see 3D at all because of vision loss in one eye or because of the loss of the muscular ability to converge and focus both eyes on any point on their visual field. One in six people have stereo impairment who might have convergence challenges like strabismus, or asymmetric visual acuity.